Cameron Simpson wrote: > About the only time I do this is my personal "the()" convenience > function: > > def the(list, context=None): > ''' Returns the first element of an iterable, but requires there to be > exactly one. > ''' > icontext="expected exactly one value" > if context is not None: > icontext=icontext+" for "+context > > first=True > for elem in list: > if first: > it=elem > first=False > else: > raise IndexError, "%s: got more than one element (%s, %s, ...)" \ > % (icontext, it, elem) > > if first: > raise IndexError, "%s: got no elements" % icontext > > return it > > Which I use as a definite article in places where an iterable should > yield exactly one result (eg SQL SELECTs that ought to get exactly > one hit). I can see I wrote that a long time ago - it could do with some > style fixes. And a code scan shows it sees little use:-)
A lightweight alternative to that is unpacking: >>> [x] = "" Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: need more than 0 values to unpack >>> [x] = "a" >>> [x] = "ab" Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: too many values to unpack -- http://mail.python.org/mailman/listinfo/python-list
